1. Field of the Invention
The present invention relates to a suspension controller for a vehicle, particularly to a suspension controller for restraining the vibration of the body of an automobile due to the irregularity of road surface during movement of the automobile.
2. Prior Art
The suspension of a vehicle is controlled depending on the condition of the surface of the road or the moving condition of the vehicle to alter the characteristic of the suspension as occasion demands, not only to improve the feel of the ride of the vehicle but also to assure good operating properties and good stability. For example, altering the constant of the air spring of a suspension depending on the condition of the road surface, altering both the constant of an air spring and the damping force of a shock absorber, and increasing the damping force of a shock absorber at a prescribed vehicle height were proposed in Japan published unexamined patent application Nos. Sho-59-26638, Sho-59-23712, and Sho-58-30542, respectively. In addition, altering the height of a vehicle was proposed in Japan published unexamined patent Nos. Sho-57-172808 and Sho-59-23713. In the above-mentioned control, the spring constant of the suspension, the damping force and the vehicle height are regulated when the rise or fall of the front part of the vehicle is detected by a braking sensor and an acceleration sensor, to improve the feel of the ride of the vehicle.
However, in the conventional control, if the characteristic of the suspension is altered because a vehicle height sensor has judged that the amplitude of the vibration of the vehicle height is larger than a prescribed value, the amplitude is changed due to the alteration of the characteristic of the suspension even though the movement of the vehicle on the road surface remains unchanged. For that reason, there is a problem that the condition of the road surface is mistakenly judged to have changed after the control, though the vehicle is moving on a road surface which is basically unchanged. In addition, if the characteristic of the suspension is altered to increase the height of the vehicle, the center of gravity of the vehicle is raised, the rolling sway of the vehicle is increased, and the amplitude of the vibration of the height of the vehicle is made larger than that when the vehicle height is lower. For that reason, there is another problem that the characteristic of the suspension is overcontrolled even if the protrusion and sinking of the road surface has actually become smaller. Furthermore, when the suspension is controlled to increase its spring constant or damping force, the amplitude of the vehicle height vibration is made smaller than before application of the control, to cause a problem of hunting in which, though the road surface condition has not changed, the characteristic of the suspension is returned to the original state and then altered again.